Rules straddle the border between static and dynamic C++. In effect, a rule
transforms compile-time polymorphism (using templates) into run-time polymorphism
(using virtual functions). This is necessary due to C++'s inability to automatically
declare a variable of a type deduced from an arbitrarily complex expression
in the right-hand side (rhs) of an assignment. Basically, we want to do something
like:

T rule = an_arbitrarily_complex_expression;

without having to know or care about the resulting type of the right-hand side
(rhs) of the assignment expression. Apart from this, we also need a facility
to forward declare an unknown type:

T rule;
...
rule = a | b;

These limitations lead us to this implementation of rules. This comes at the
expense of the overhead of a virtual function call, once through each invocation
of a rule.

Multiple declaration

Some BNF variants allow multiple declarations of a rule. The declarations
are taken as alternatives. Example:

r = a;
r = b;

is equivalent to:

r = a | b;

Spirit v1.3 allowed this behavior. However, the current version of Spirit no
longer allows this because experience shows that this behavior leads to
unwanted gotchas (for instance, it does not allow rules to be held in containers).
In the current release of Spirit, a second assignment to a rule will simply
redefine it. The old definition is destructed. This follows more closely C++
semantics and is more in line with what the user expects the rule to behave.

Sequencing Syntax

The comma operator as in a, b seems to be a better candidate, syntax-wise. But
then the problem is with its precedence. It has the lowest precedence in C/C++,
which makes it virtually useless.

Bjarne Stroustrup, in his article "Generalizing
Overloading for C++2000" talks about overloading whitespace. Such a
feature would allow juxtapositioning of parser objects exactly as we do in (E)BNF
(e.g. a b | c instead of a >> b | c). Unfortunately, the article was dated
April 1, 1998. Oh well.

Forward iterators

In general, the scanner expects at least a standard conforming forward iterator.
Forward iterators are needed for backtracking where the iterator needs to be
saved and restored later. Generally speaking, Spirit is a backtracking parser.
The implication of this is that at some point, the iterator position will have
to be saved to allow the parser to backtrack to a previous point. Thus, for
backtracking to work, the framework requires at least a forward iterator.

Some parsers might require more specialized iterators (bi-directional or even
random access). Perhaps in the future, deterministic parsers when added to the
framework, will perform no backtracking and will need just a single token lookahead,
hence will require input iterators only.

Why are subrules important?

Subrules open up the oportunity to do aggressive meta programming as well because
they do not rely on virtual functions. The virtual function is the meta-programmer's
hell. Not only does it slow down the program due to the virtual function indirect
call, it is also an opaque wall where no metaprogram can get past. It kills
all meta-information beyond the virtual function call. Worse, the virtual function
cannot be templated. Which means that its arguments have to be tied to a actual
types. Many problems stem from this limitation.

While Spirit is a currently classified as a non-deterministic recursive-descent
parser, Doug Gregor first noted that other parsing techniques apart from top-down
recursive descent may be applied. For instance, apart from non-deterministic
recursive descent, deterministic LL(1) and LR(1) can theoretically be implemented
using the same expression template front end. Spirit rules use virtual functions
to encode the RHS parser expression in an opaque abstract parser type. While
it serves its purpose well, the rule's virtual functions are the stumbling blocks
to more advanced metaprogramming. Subrules are free from virtual functions.

Exhaustive
backtracking and greedy RD

Spirit doesn't do exhaustive backtracking like regular expressions are expected
to. For example:

*chlit_p('a') >> chlit_p('a');

will always fail to match because Spirit's Kleene star does not back off when
the rest of the rule fails to match.

Actually, there's a solution to this greedy RD problem. Such a scheme is discussed
in section 6.6.2 of Parsing Techniques: A Practical
Guide. The trick involves passing a tail parser (in addition to
the scanner) to each parser. The start parser will then simply be: start
>> end_p; (end_p is the start's tail).

Spirit is greedy --using straight forward, naive RD. It is certainly possible
to implement the fully backtracking scheme presented above, but there will be
also certainly be a performance hit. The scheme will always try to match all
possible parser paths (full parser hierarchy traversal) until it reaches a point
of certainty, that the whole thing matches or fails to match.

Backtracking
and Greedy RD

Spirit is quite consistent and intuitive about when it backtracks and
to where, although it may not be obvious to those coming from different
backgrounds. In general, any (sub)parser will, given the same input, always
match the same portion of the input (or fail to match the input at all).
This means that Spirit is inherently greedy. Spirit will only backtrack
when a (sub)parser fails to match the input, and it will always backtrack
to the next choice point upward (not backward) in the parser structure.
In other words abb|ab will match "ab", as will a(bb|b), but
(ab|a)b won't because the (ab|a) subparser will always match the 'b' after
the 'a' if it is available.

--Rainer Deyke

There's a strong preference on "simplicity with all the knobs when you
need them" approach, right now. On the other hand, the flexibility of Spirit
makes it possible to have different optional schemes available. It might be
possible to implement an exhaustive backtracking RD scheme as an optional feature
in the future.